We studied 5-hydroxy tryptamine type 3 (5-HT3) receptors
transfected in tsA-201 cell line to examine serotonin-induced
whole cell currents. Using the site-directed mutagenesis
technique, we individually mutated each residue in the
membrane-spanning M2 segment to histidine. A high proportion of
tsA-201 cells cotransfected with the cDNAs of 5-HT3R and CD8
produced large amplitude responses (0.5–7.0 nA) to
serotonin. The dose-response curve of wild-type (WT) receptor
ranging from 0.5 to 500 μmole increases its Kd
values, and Imax
of 5-HT3R falls at low external pH as if
protonation of an acid group is enough to block the channel.
Lysine at position 281, a basic residue, is more susceptible to
acidification-induced blockade of the 5-HT3R channel.
Dose-response curves of K281S (replacing lysine at the 281
position with serine) at different pH are not significantly
modulated, and histidine substitutions at the three consecutive
positions 293, 294, and 296 eliminate the pH block of the channel.